In order to unlock your ZTE T790 Telstra device, download and install the latest version of SigmaKey. After that, connect the device to your computer using the USB cable, press the Volume Up + Volume Down keys and launch SigmaKey. Press the Unlock button, select the ZTE device from the Serial Port drop-down list and then press Direct Unlock. Wait several seconds and after the process is complete, restart your phone. If you have issues, you can use Google search engine to find many unlocking tutorials for your device model.